As well known, a standard configuration for a gas turbine envisages a plurality of vanes solidly connected to an outer casing, or vane carrier, which surrounds a rotating shaft guided by blades mounted thereon. In particular, each vane comprises an airfoil which is connected to a vane platform, which is in turn retained into the outer casing. As hot combustion gases pass through the casing to drive the rotating shaft, vanes experience high temperatures.
Generally a vane can be fixed to the outer casing at its outer diameter, in a cantilever fashion, or at its outer and inner diameters (the latter design known as rocking vane).
With reference to FIG. 1, it is schematically shown a stator vane 100 in cantilevered design according to the state of the art, wherein the vane 100 includes an airfoil 103 mounted on a vane platform 104 comprising a leading edge hook 102 and a trailing edge hook 101, which are in turn mounted in a vane carrier 105. Axial and circumferential fixation may be operated either on the leading or trailing edge hooks 102, 101.
With reference to the following FIG. 2, it is shown a stator vane 200 in a “rocking vane” configuration, according to the prior art. In this case, a vane 200 includes an airfoil 203 mounted on a vane platform 204, which in turn comprises an outer single hook 201 fitted into a vane carrier receiving portion 205. Hook 201 provides outer axial, circumferential and radial support and translates axial, radial and circumferential vane loads into the vane carrier 205.
Further, vane 200 is supported axially at its inner diameter 202 by an inner structural component 208, which provides inner axial support. The component 202 is fitted into the vane carrier 205, as schematically indicated in the figure. The vane 200 is pushed against the outer and inner axial vane carrier supports 205, 208 by the axial gas load applied to the airfoil 203.
Due to different thermal expansion of the structural parts of a gas turbine engine in transient modes, the inner and the outer axial supports 205, 208 of the vane 200 will vary axially relative to each other.
This will cause the vane 200 to tilt relative to the vane carrier 205 as shown in FIG. 3. Moreover due to thermal stress in the vane itself, hook 201 may bend in any direction.
In general, according to the teachings of the prior art, vane 200 provides a circumferential hook 201 having a cylindrical space on the outer side and a plane surface on the inner side. The receiving groove in the vane carrier 205 provides outer and inner cylindrical surfaces which create a surface contact 206 at the outer side and an axial line contact 207 at the inner side, as shown in FIG. 4. In order to prevent undesired tilting of the hook 201 in circumferential direction within the receiving groove of the vane carrier 205, clearance between vane hook 201 and vane carrier 205 is typically kept as small as possible. Particularly for rocking-type of vanes, there are several drawbacks of the prior art.
Firstly a thermal deformation of the hook (e.g. bending) may jam the vane inside the groove. This will introduce high forces into the vane or the carrier, which results in a reduced lifetime.
A possible partial solution to such problem might be increasing the clearance, however this may allow for a considerable tilting of the vane in the circumferential direction. Moreover the vane shall be free to rotate around the hook about a few degrees (+/−5° max.) to compensate relative outer and inner support movements which is not possible with an axial line contact and surface contact.